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Return needed?

I'm hoping someone can help me with this --

I have an upstairs bedroom measuring about 300 SQ ft with only one floor mounted duct. Duct volume is quite low, and there seems to be no return in the room. The room has double doors opening to a hallway, opposite the wall where the floor duct is located.

When the doors are left partially or full open, the room temps pretty well match the rest of the floor, but when the doors are closed, the room gets quite hot in summer and cool in winter. I assume this is due to the fact that there is no return and that the there is very low air volume from the one duct in that room. There is a good 3/4" of clearance from the bottom of the door to the floor.

I was wondering if simply cutting a 30 x 8 hole in the wall (next to the doors to the hallway) and installing a return air grill on both sides would give me enough negative pressure to get the temperatures closer to the rest of the floor, or do I need to install an air exchanger? Volume in the ducts are low on the entire top floor, but the other two rooms and hallway seem to be just fine. Installing a heat pump to cover the top floor would be ideal, but I already have 3 in the home and do not wish to incur another expense such as that. What would be my best options here? I have had a few HVAC contractors in to look at some of the issues, but they weren't much help...

The home is about 6400 SQ ft including the basement, ground and top floor. There is an air exchanger - but only for the bathrooms. The main heat pump is a 5-ton, with a minisplit and the garage and another minisplit for one office on the ground floor. Aside from that one room, there are no other real heating / cooling issues.

what is above the room? attic space? where is the hall return? in the floor or the ceiling? 300Sq ft, is a large room, additional supply ductwork may be needed to offset the radiant heat from the attic space. since the doors left open seems to properly condition the room, adding in a return could help, as could a air balance duct from the room to the hallway. low pressure is sometimes thought of as low volume of air, but since the room properly conditions with the doors open, it's unlikely to be low volume.

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The room measures 14 x 14 with 9 ft ceilings. Attic is located right above. I did locate one return and there is indeed airflow, but the room is stil hot with the doors closed. It's about 3 degrees warmer than the ground floor with the doors closed and about 1 degree warmer with the doors open. There are 2 returns in the hallway - one next to the double doors and one about 18 ft further down, both located on the wall just below the ceiling. I don't want to break walls to solve the problem, if it can be avoided.

You've basically already found the solution that you make sound is working for you and that is leaving the door open. Making a permanent opening through the wall to accomplish the same thing as the door being open (as long as the functional square inches of opening were the same as the door open to the width needed to accomplish the balance in temp) will get you the same results. Pass grills (which is what you're describing) are used all the time when a central return is responsible for returning air from spaces that have supplies, but have a door, that when closed can block the path of air needing to be returned to the equipment. Are pass grills the best solution? Let's just say they are "cost effective". Is a dedicated return in each space better? Better is a relative condition that, in either form, requires the entire system being balanced correctly to deliver the required amount of air to each space. Air movement = energy movement, either adding heat or removing it.

Keep in mind that registers themselves are restrictive and that even though it is physically a 30x8 you can figure that the actual "hole" that the air will see is normally half that much.

Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

Thanks - though I won't be able to push as much air through a grill as having the doors open, I will try that for now and see if I get gains. Cost is obviously marginal to try. I will look at adding a ceiling fan to get more circulation as well. Thanks for the speedy and informative replies fellas!